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Homework Help: Charge on a Wire

  1. Feb 26, 2010 #1
    1. The problem statement, all variables and given/known data
    The electric field 4.80 cm from a very long charged wire is ( 2100 N/C,toward the wire).
    What is the charge (in nC) on a 1.0-cm-long segment of the wire?

    2. Relevant equations

    E=1/(4*pi*eo) * Q/ri^2 * cos (theta)

    3. The attempt at a solution

    I put in 9*10^9 for 1/(4*pi*eo)
    I put in (.048^2+.01^2) for ri^2 and for cos theta i put .01/4.8

    I thought this equation would have worked with the old plug and chug method, is this the right equation for this situation?

  2. jcsd
  3. Feb 26, 2010 #2


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    Homework Helper

    No, that's not correct. Think about what the equation means and you'll see why.

    To start off, assume that the line of charge has a charge density of lambda and use that to find an equation giving the electric field in terms of r. Then plug and chug into THAT equation.
  4. Feb 26, 2010 #3
    Ah. 2(lambda)/(4*pi*eo*r)
    lambda = Q/L
    Got it.
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